This invention relates to field emission devices and more particularly a field emission device utilizing a microchannel gain element to multiply electron emission.
It has become increasing common in the construction of image-generating devices to utilize a field emission device or xe2x80x9ccoldxe2x80x9d cathode as a source of electrons for exciting a surface that generates a visible light. The cathode is placed in a vacuum enclosure and electrons are emitted from the cathode by action of a strong electric field adjacent the cathode. The electric field results from the cathode""s geometry and from the use of a collector or anode adjacent the cathode. The anode is biased with a strong positive electric potential relative to the cathode. The emitted electrons generate a beam that passes between the cathode and anode. This beam can be modified by a third electrode known as a gate or grid. Further electrodes can also be added within the vacuum space between the cathode and the gate to further modify the electron beam. The resulting assembly can be termed a triode, tetrode, pentode, or the like, depending upon the number of electrodes present, in addition to the anode and cathode.
A generalized field emission device according to the prior art is detailed schematically in FIG. 1. The cathode 20 includes an emitter section 22 having a sharp emitter point 23. The emitter 22 is located behind a gate 24 having an opening 26 through which electrons pass to strike an anode 28. The cathode 20, gate 24 and anode 28 are separated from each other spatially and are enclosed in an evacuated envelope 30. Field emission or xe2x80x9ccoldxe2x80x9d cathodes such as cathode 20 are known generally to those who are skilled in the art. Current methods of fabricating and characterizing these devices is described in a special issue of the Journal of Vacuum Science and Technology B. Microelectronics and Nanometer Structures, Second Series, vol. 12, no. 2. March/April 1994and is hereby expressly incorporated herein by reference.
When the anode 28 of FIG. 1 is employed as a display device (such as in a flat panel display) a phosphor is provided integrally to the anode. The phosphor is sensitive to electron excitation and emits visible light. Likewise, the cathodes 20 are organized in arrays that are addressable to generate an image. Where the structure of FIG. 1 is to be employed in electron microscope, the object to be analyzed is part of the anode 28.
In a display device, the brightness of light emitted by the phosphor screen depends upon the density of electrons that strike a given area of the screen and the energy derived from the voltage difference between the anode and the cathode. The brightness also depends upon the efficiency of the phosphor in converting the energy of electrons to photons of visible light. Typically, to achieve a high current density, and therefore a high-brightness in a display device the electric field that extracts electrons at the emitter 22 must be in the range of 107 Volts/centimeter. This high field strength invariably leads to cathode tip damage by erosion. Such erosion makes the emitted beam unstable in the short term and impairs long-term reliability. Thus, erosion makes construction of flat panel displays using the structure FIG. 1 limited in brightness. Cathode damage and beam instability are also encountered in developing electron sources for integrated circuit lithography and electron microscopy.
IBM Research Report RC19596 (86076), Jun. 3, 1994, entitled xe2x80x9cEmission Characteristics Of Ultra-Sharp Cold Field Emittersxe2x80x9d (now published in the Journal of Vacuum Science and Technology. B12(6), p.3431. Nov./Dec. 1994) by Ming L. Yu, et al describes cathode deterioration at high current densities due to ohmic power dissipation, electron static stress and ion etching from residual gases in the evacuated space. The report also relates to long term and short term current instabilities during electron emission at high current densities.
It is desirable that electrons emitted by each cathode be as concentrated as possible when used in a display device. Transverse spreading of electrons generates larger pixels and, thus, lowers the density of pixels on the screen. Spreading of the electron beam also results in noise and reduced contrast, since electrons strike the phosphor in an area other than the intended pixel. To combat spreading, displays according to the prior art have located the anode/phosphor screen as close to the cathode array as necessary to attain desired resolution, contrast and signal-to-noise ratio. However, the placement of the anode and cathode in close proximity makes construction difficult and imposes design constraints that increase construction cost and/or degrade performance.
While the cathode structure FIG. 1 can also be utilized in a color display device, by providing phosphors of three different colors (red, green and blue, for example) in a cluster with three independent emitters, the above-described problems remain. Additionally, accurate control of color generated by the three, clustered, subpixels must also be maintained. Electron beam spread and instability complicate the maintenance of good color fidelity.
In order to overcome the problems inherent in a field emission cathode operating at high current, it is contemplated that the cathode can be operated at a substantially lower current. However, a lower current, while reducing erosion and increasing electron beam stability, does not generate a beam of sufficient density. The resulting weak beam is typically insufficient to cause currently available phosphors to emit a bright visible light. The emitted electron beam is also insufficient to perform detailed electron microscopy or electron beam lithography with improved performance and results.
In the field of lithography for producing integrated circuits, in particular, the goal is to create complex, microscopic circuit patterns on a semiconductor wafer substrate composed, for example, of silicon or gallium arsenide. To accomplish the patterning process, the wafer is first covered with a protective layer of polymeric, light-sensitive material, known as a photoresist. The photoresist is dried by a baking or xe2x80x9ccuringxe2x80x9d process, and then is exposed selectively to light in a pattern defined by a xe2x80x9cmaskxe2x80x9d having transparent and opaque areas analogous to the desired circuit pattern. Where the mask is transparent, light passes through, onto the substrate, exposing each portion of the circuit pattern. The exposed circuit pattern on the substrate undergoes a chemical change in response to the light, while the remaining unexposed photoresist is unchanged chemically. When the photoresist is xe2x80x9cdeveloped,xe2x80x9d in a manner similar to photographic film, a defined circuit pattern on the substrate at a microscopic level results. The developed photoresist enables selective processing of the semiconductor wafer to produce a layered structure with a variety of conducting, semiconducting and insulating media that define the finished circuit.
A mask can be placed directly on the surface of the substrate to produce a 1:1 scale image on the substrate. This is termed xe2x80x9ccontact lithograplhy.xe2x80x9d since the mask essentially contacts the substrate. Alignment of the mask is important in a contact arrangement, and a special contact aligner is used for this purpose, since the mask must maintain alignment with the substrate within a close tolerance range as the substrate undergoes several different layers of lithography.
Other techniques for producing circuit patterns on a substrate entail the use of xe2x80x9cproximity alignment,xe2x80x9d xe2x80x9cprojection alignment,xe2x80x9d or xe2x80x9cstep-and-repeat alignment.xe2x80x9d The most widely employed in the semiconductor industry is currently the step-and-repeat technique. A xe2x80x9cstepperxe2x80x9d mechanism transfers the image from a relatively larger 5xc3x97 or 10xc3x97 scale (for example) mask to the substrate using, a reduction process that employs a sophisticated optical system. The substrate is moved in increments, or xe2x80x9cstepped,xe2x80x9d under the optical system as the process proceeds incrementally to expose the entire wafer. Typically, lithography is one of the slowest steps in the fabrication of wafers. The wafer throughput of a current stepper is on the order of one hundred wafers per hour.
In each of the above-described photoresist exposure techniques, visible or ultraviolet light is generally used to as the exposure agent. The wavelength of light poses a general limit on the width of circuit lines on an exposed substrate. In general, line width of no less than 180 nm can be produced using light in the ultraviolet region of the spectrum. Conversely, electron beams can be focused to a much-finer extent. Using an electron beam, lines having a width of 20 nm or less can be achieved, essentially an order of magnitude finer than that possible with photolitlhographic processes. This enables much denser packing of transistors onto a given area enabling higher performance in a smaller package.
Conventional electron beam lithography techniques are limited in that they require the substrate to be exposed by the beam in a serial fashion by scanning the beam over the substrate, typically in a line-by-line (raster-style) manner. The scanning time involved is significantly longer than the step-and-repeat (stepper) method using visible or ultraviolet light.
In order to improve the throughput of electron beam lithography, research has been conducted by several investigators using arrays of parallel electron beams. The source of the electrons is typically an array of electron emitters. Such emitters have generally comprised xe2x80x9ccoldxe2x80x9d cathodes, since hot cathodes (heated wires) or xe2x80x9cwarmxe2x80x9d cathodes (Schottky emitters) generate excess heat when closely packed together. With a sufficiently large array of emitters and a corresponding array of electron beam lenses to focus the output of the emitters it has been possible to increase the throughput of patterned semiconductor wafers substantially.
In operation, the array of emitters is matrix-addressable. That is, a given field emission device in the array can be individually addressed via row and column drivers. A typical field emitter consists of a cathode tip(s) and a gate or xe2x80x9cgrid.xe2x80x9d When a particular row and column in the array are energized by the application of a suitable voltage, the emitter at the intersection of the row and column addresses is activated, and emits electrons. The electron beam lens corresponding, to the location of the emitter focuses the emitted electrons onto an anode, typically with an energy of 1,000 to 100,000 electron Volts. In this example, the anode is a wafer substrate coated with a resist that is sensitive to the impacting electron beam. Thus, a focused spot pattern is formed on the wafer. If the wafer is moved relative to the incident electron beam, then a focused line is patterned on the resist of the wafer. Likewise, each emitter can independently pattern a line in the substrate. It follows that, if two adjacent emitters in a row are activated simultaneously, then the motion over the spacing between the emitters produces a line twice as long as that produced by a single emitter. Likewise, if all the emitters in a row are activated simultaneously a line is patterned in the substrate that corresponds to the full length of the row of emitters even though the total motion to produce the line is limited only to the distance between emitters. In this parallel method of patterning the wafer it is hence, possible to improve the wafer throughput substantially beyond what is possible in the serial method described above.
However, in order to achieve sufficient electron beam current to expose the pattern in the resist-coated wafer, the field emission cathodes of the prior art are operated at current levels that cause short-term instability and poor long-term reliability. Thus, while the parallel method of electron beam lithography is attractive for its potentially high throughput, and for obtaining linewidths not possible with photolithography it is limited in application by significant reliability considerationxe2x80x94considerations that clearly do not lend the technique to mass-production semiconductor fabrication processes.
Accordingly, it is an object of this invention to provide a field emission device structure that enables operation of a field emission cathode at a lower current without the corresponding loss in electron beam strength. The structure should be capable of tightly focusing the generated beam even when the anode is located at an increased distance from the cathode. In a display, the cathode should generate a beam that triggers sufficient visible light emission in conventional phosphors. The generated beam should be stable and the cathode should have increased reliability due to reduced erosion. In electron beam lithography, the generated beam or beams should enable rapid and finely focused exposure of a circuit pattern on a semiconductor substrate generally coated with a resist that is reactive to the impacting electrons.
A field emission device, with a microchannel gain element according to this invention, provides an array of field emission or xe2x80x9ccoldxe2x80x9d cathodes located on a substrate. The cathodes can be addressed individually or in groupings that correspond, in an image display device, to pixels. A gate or grid system is typically located in conjunction with the cathode array and is driven at a constant voltage with a superimposed modulating voltage to control emission of the cathodes. The cathodes are located in an evacuated space so that, upon application of a predetermined voltage, an electric field enables emission of electrons from individually addressed cathode emitters. The emitted electrons pass through the gate or grid and, according to this embodiment, enter a microchannel gain element. The microchannel gain element includes a pair of opposing anode sides that are driven at a voltage difference. The microchannel gain element also includes a plurality of microchannels that correspond to each of the cathodes. The microchannels include a secondary-electron-emissive layer therein. When electrons from each of the cathodes strike the emissive layer, the emissive layer generates additional electrons. A cascade effect ensues as electrons pass down each of the microchannels, and the resulting electron beam that exits each of the channels typically has a gain in a range of 100-200 (or more) relative to the entering electron stream. The use of a gain element enables the current generated by each of the cathodes to be substantially lower for a given anode current. This lower current adds to electron beam stability and reduces erosion of cathode emitters. A substantially conventional anode is located adjacent the exit of the microchannel structure.
The anode, in an image device, can comprise a glass, or other transparent material, plate having a phosphor and a thin metallic film thereon. Alternatively, in an electron microscope, the anode can include the object being viewed. In a lithographic process, the anode can include a semiconductor wafer coated with a sensitive resist. In particular, the anode can comprise a semiconductor substrate having a reactive resist, and an electrostatic lens structure can be employed to focus the beams to produce a pattern on the substrate according to a predetermined geometric design. The pattern is provided to an address controller that selectively activates the emitters and gating structure, or xe2x80x9cgrid,xe2x80x9d to reproduce the mask pattern in a single pass, or a sequence of passes. The lens structure can comprise a plurality of plates driven by predetermined voltages and stacked with apertures aligned with the emitters and outlets of the microchannel plate. A plurality of stacked microchannel plates can be provided with channels aligned to further increase the gain of the beam entering the lens structure.
In all of the above-described embodiments, the use of a microchannel gain element enables generation of more-stable electron beams and increases cathode reliability and life. The microchannel plate according to this invention can be formed in a variety of ways from a variety of materials. The construction lends itself to improved electron beam lithographic processes and enables the formation of high resolution pixels and/or circuit pathways.